A sewing machine generally comprises a worktable overhung by an arm from the head or working end of which a presser foot is resiliently mounted and in which a needle bar is reciprocatable to periodically thrust the needle through a fabric advanced across this worktable over a stitching plate to form a row of stitches in the fabric.
The worktable is provided with a hole through which the needle can pass and windows or openings through which dogs, driven by a mechanism below the worktable and synchronized with the needle through which the intermittent movement of the dogs can advance the fabric by engagement with the underside of the workpiece.
Thus, while the dogs, which can have teeth or other formations along the upper surface to positively grip the underlying fabric layer, tend to remove this underlying fabric layer in the direction of advance of the workpiece, the upper layer of the latter is frictionally engaged by the presser foot and may be relatively retarded.
It has been found that the positive advance of the lower layer, coupled with retardation of the upper layer, can result in irregular stitch formation, in wrinkling, in bunching and even in relative offsetting of the upper and lower layers so that they may be disaligned or rendered noncoextensive.
To prevent such relative shifting of the layers of a workpiece it has already been proposed to provide an upper feed member which engages the upper layer and is intended to advance the latter synchronously with the advance of the lower layer by the dogs. In German Pat. No. 10 61 603, for example, this upper member is an intermittently rotated feed roller or wheel connected to a mechanism for stepping the rotation of the wheel or roller synchronously with the movement of the feed dogs.
A difficulty with such systems, however, is that the stepping rotation of this wheel or roller is not adjustable as to angular velocity, stepping angle and like parameters as is important for high versatility of the mechanism.
In other fields it is known to provide transport rollers which are angularly oscillated and in which the angular or stepping displacement, represented by the angle .alpha. can be steplessly varied during the running of a sewing machine.
In earlier systems of this type, there were a number of problems. For example, when the feed roller was driven by a crank drive, the intermittent movement was generally transmitted to the roller from the crank drive by an undirectional clutch allowing the crank to return after each stroke, this unrotational clutch or like member being provided with a brake. The purpose of the brake was to prevent, at high sewing speeds, an overrotation during the return stroke so as to ensure, to the greatest extent possible, a constant stepping angle .alpha. of the mechanism. This brake, however, introduced a drag to the system which increased the power requirements and decreased efficiency.
Another disadvantage of the earlier systems described was the result of inertial forces which, at high sewing rates and/or with relatively great stitch length and hence fabric advance steps resulted in an increasing overrun even when the brake was provided. This overrun tended to increase still further during long periods of operation and resulted in a noticeable decrease in sewing quality and output.
German patent document 17 50 498 discloses a transmission or drive for a rotary member engageable with the fabric on a sewing machine table which superimposes an oscillating angular displacement upon a continuous rotary displacement but without any possibility of stepless control of the stepping angle to compensate for variation in stitch length and the degree of advance by the feed mechanism below the work table, during operation of the sewing machine.